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I have been reading a lot lately about people hating on SOAP based web services. As a whole, the Web is moving more toward REST based APIs. This post is to make a case for WSDL and SOAP based web services.

Don’t get me wrong. I’m a huge fan of RESTful Web Services. I use them in many places where they make sense in the software I develop. I am not writing this post to say that SOAP/WSDL based web services are better than the REST style but I intend to point out some of the things that a WSDL does better.

It all comes down to perspective. Who is going to be consuming the Web Service? Is the consumer going to be a human or a machine? Let’s look at each case.

Humans Consuming Web Services. By humans consuming web services, I mean a programmer sits down and writes some code to use a web services. The common example of this is a developer from a website using the Flicker, Twitter, Google Maps or Facebook API to integrate with their site. Even in the business world, when a user needs to write a piece of code to connect two things together, REST is the clear winner. REST style web services are easier to work with and usually result in much cleaner code.

Machines Consuming Web Services. By machines consuming web services, I’m referring to software that interprets and leverages the web services. The primary examples for this are Business Process Management and Runbook Automation Software. This happens to be the area in which I develop software. In this space the goal is to allow machines to interpret the Web Services (or other technologies) and allow the user to just map from service to service. The user needs to know nothing about the transport or how the service function themselves as the machine is responsible for all of that. This type of software is typically used in the business world and not on the Web.

For this type of software, a strictly defined specification (a WSDL) provides a valuable tool to the software that needs to interpret it. A REST web service may be easier for a human to use by reading documentation, but we have yet to make a computer read documentation and produce results. Also, many REST style web services have XML formats that cannot be expressed in a Schema. This may be fine for a human, but XML Schemas provide an easy way for a computer to consume and understand the XML format. Yes, XML Schemas are very wordy and difficult to read by a human. They are also a pain to write properly. Even with their limitations, they do a good job of defining XML in a way that a machine can interpret them.

I see a lot of newer programmers who have only ever done work on the Web claim that REST is the best choice in all cases. We have graduated beyond SOAP to the superior technology. Even Joel Spolsky mentioned it on episode 64 of stackoverfow. REST as a superior technology is simply not true. REST may be the best choice for the Web but there are many other uses for Web Services besides the Web.

REST is getting closer to what WSDL has to offer. With WSDL version 2.0 or WADL you can define REST style web services. Maybe in a few years things will be different. Maybe we will get to the point where REST really is better than traditional web services. But we are not there yet.